Tannery Effluent The global leather industry produces about 18 billion square feet of leather a year (2003 data)with an estimated value of about $40 billion. Developing countries now produce over 60% of the world's leather needs. About 65% of the world production of leather goes into leather footwear, and the global production of footwear is estimated at around 11 billion pairs (worth an estimated $150 billion at wholesale prices). Conversion of rawhide into leather (an unalterable and imputrescible product)requires several mechanical and chemical operations involving many chemicals in an aqueous medium, including acids, alkalis, chromium salts, tannins, solvents, auxiliaries, surfactants, acids, and metallorganic dyes; natural or synthetic tanning agents; sulfonated oils, and salts. The quantity of effluent generated is about 30 L for every kilogram of hide or skin processed. The total quantity of effluent discharged by Indian tanneries is about 50,000 mg/day and contains high concentrations of organic pollutants. Tannery effluents can be divided into three types based on the operations carried out in the three different sections (Fig. 12-1)" 9 Unhairing and liming wastewater with high sulfide and lime content and high pH [accounts for 45% of the effluent volume and contributes to 30% of the overall biological oxygen demand (BOD) and chemical oxygen demand (COD)]. 9 Tanning wastewater with high salinity and high chrome levels. 9 Retanning, dyeing, and fat liquoring wastewater (accounts for --~20% of the total COD). The first area of operations is the beam house in which the raw hides and skins are cleaned and prepared so that the hides are more pliable, attractive, and useful. The operations include siding, trimming, washing, soaking, fleshing, and unhairing. The last operation involves treatment with lime and sodium sulfide as the primary chemicals to dissolve the hair. These wastewaters are highly alkaline (pH of 10 to 12). Raw leather is made up of three main layers. The upper layer (epidermis) contains hair, glands, muscles, etc. 133
134 Biotreatment of Industrial Effluents
Third Phase :
Liming Washing First phase 9 Beam house Washing after fleshing
Trimming & shaving
Deliming & bating
Second phase : Tanning
FIGURE 12-1. Various steps in conversion of hide to leather.
The middle layer (corium), which is useful and constitutes the leather product, is made to react with the tanning agent, while the bottom layer is removed by mechanical means. Removal of the upper layer is carried out through liming and bating processes. The skin is rehydrated by soaking and removing the globular proteins. Hair is removed from the skin by the reductive liming process. Two important processes take place during this step mhydrolysis and chemical reduction of keratin. Hydrolysis occurs in an alkaline medium, and lime acts as a buffer, maintaining the pH around 12.5. While globular proteins are easily solubilized, keratin and collagen are resistant to hydrolysis. Reduction of keratin is achieved by the use of alkali sulfide salts. Elastin, another strong fibrous protein that is not soluble under the conditions of liming, is removed using enzymes (the bating process). Pickling is another pretanning step where some globular proteins are removed with the aid of pretanning agents. The second area of operations is the tanyard in which a durable material is produced from the animal hides or skins. The proteinaceous matter in the hides is made to react with the tanning agent for stabilization. This is accomplished by using synthetic tanning agents containing trivalent chromium salts, aluminum, zirconium, etc., or using vegetable tannins extracted from the bark of certain trees; 90% of tanning is done using the former. These operations are carried out in an acidic medium, and the wastewater generated usually has a pH in the range of 2.5 to 3.5. The tanning process stabilizes the skin structure by forming transverse bonds among its fibers. In the case of mineral tanning agents, the tanning agent blocks the carboxylic groups
(or in the case of vegetable tanning agents, the amine groups) and joins the proteinaceous colloid, thus increasing the crosslinking of the collagen fibers. The resulting stabilized leather material cannot be degraded by physical or biological means. Of the tanning agents added, 15 % does not get fixed to the leather and is discharged with the effluent. The third set of process operations involve retanning and wet finishing, which gives the tanned hides special or desired features such as bleached appearance, added coloring, lubrication, or further tanning for finished leather properties. These operations usually do not alter the pH of the wastewaters. Solid waste is generated, including trimmings, degraded hide, and hair from the beam house, amounting to 70% of the wet weight of the original hide. The tannery wastewater has a very high salinity. The main contributors are 30% chlorides from the pickling bath (where skins are prepared with salts and acids prior to adding tanning agent) and 60% sulfates from the tanning bath. Hydrogen sulfide is released during dehairing, and ammonia is released during deliming. Characteristics of the effluent released from a tannery producing full chrome upper leather from dry salted bovine hides is given in Table 12-1; the effluent from an industrial district housing several tanneries is given in Table 12-2. TABLE 12-1 Characteristics of Effluent Released by a Tannery Treating Bovine Hide pH SS TS Chromium COD BOD
7.5-9.0 1,5004,000 mg/L 29,00045,000 mg/L 100 mg/L 5,000-10,000 mg/L 1,500-2,000 mg/L
TABLE 12-2 Wastewater Characteristics of Tannery Effluent (Turkey) Characteristic, mg/L
Total COD Soluble COD BOD5 SS VSS Total nitrogen Organic nitrogen
5,094 2,336 1,760 2,229
2,216 1,187 958 794 506 226 62 164 5.1 41 27
358 223 135
Total phosphorus Total chromium (Cr3+) Sulfur
136 Biotreatment of Industrial Effluents Nitrogen exists in leather tanning wastewaters as ammonia and organic nitrogen and is present in both particulate and soluble forms. All nitrogen originating from the bovine leather processing plant is from soaking, liming, deliming, bating, pickling, and tanning, and the washings from these processes (Zengin et al., 2002), and the total nitrogen in the effluent is below 1%. The main source of nitrogen is from the liming step (-~7390 mg/L), followed by deliming, bating, washing, and pickling steps. The main source for protein in the effluent comes from the liming step (~13,660 mg/L), followed by the washing, deliming, and bating steps (Kabdasli et al., 2003). Physical and C h e m i c a l T r e a t m e n t Mixing all three effluents and then treating is found to be very inefficient. The unhairing wastewater is treated with oxygen in the presence of a manganese (II) salt, which acts as a catalyst to transform the sulfide to sulfate. The wastewater of the tanning process, which contains large quantities of chromium, is directly recycled or treated with alkali to precipitate chromium hydroxide, which is subsequently reused. The retanning, dyeing, and fat liquoring wastewater is treated with iron (II) sulfate to precipitate a large amount of protein and organic contaminants. Still, some contaminants such as surface active agents and fat liquors remain unaltered by a normal physicochemical treatment. The recovery of chromium from tannery waste is carried out in three ways: namely, Cr (III) extraction with sulfuric acid solution at a pH of 1, Cr(III) oxidation to Cr(VI)with H202, and Cr(VI) separation from other cations and its subsequent reduction to Cr(III). The overall Cr (III) recovery yield from sludges is about 80% (Macchi et al., 1991). Adsorption using clays such as bentonite, sepiolite, or activated carbon, or expensive tertiary treatments like the Fenton process have also been found effective in treating the final effluent (Espantaleo et al, 2003). Coagulation followed by removal of sludge is an old technique that has been effectively adapted for treating effluents of different types. The most widely used coagulants are aluminum (III) and iron (III) salts. Although vegetable tannage produces small amounts of wastewater, it has a negative impact on the primary treatments like flocculation or coagulation, and leads to increased use of chemicals (Scholz and Lucas, 2003). The wastewater from leather tanning contains organic nitrogen that comes from proteins and amino acids. These compounds have been recovered by several physical and chemical means, including addition of miscible solvents such as alcohol or acetone; addition of metal cations such a s Zn 2+, Cd 2+, Cu 2+, or bulky anions such as perchlorate, trichloroacetate; salting out using ammonium sulfate or other salts; isoelectric pH precipitation; and polyelectrolyte aggregation precipitation. Many of these techniques are not economical and cannot be used on an industrial scale. For protein removal, 50% is achieved by isoelectric pH precipitation carried out between the optimum pH interval of 2.1 to 3.8, and 60% is achieved with FeC13; 85%
ammonia removal and 50% protein removal is achieved with magnesium ammonium phosphate precipitation followed by acid precipitation of the protein (Kabdasli et al., 2003).
Biochemical Treatment Aerobic A combination of biochemical oxidation and chemical ozonation of tannery effluent has been found to yield excellent results, with the first part performed in an upflow sequencing batch biofilm reactor provided with external recycle (Iaconi et al., 2002). COD, N H 4 m N , and total suspended solids (TSS) removals were 95, 98, and 99.9%, respectively. The combined process produced very low sludge, about 0.03 kg TSS/kg COD removed, which is much lower than the values reported in the literature for conventional biological systems. Ozone helped in the mineralization of some organic substances and the partial oxidation of some others, leading to enhancement of the biodegradability of the effluent. The aerobic treatment of the beam house and tanyard wastewater substreams, followed by an oxidative treatment using ozone, and a second aerobic treatment improved the aerobic biodegradability of refractory organic compounds. Also, full nitrification was achieved during the subsequent aerobic degradation, and the remaining ammonia was completely removed (Jochimsena et al., 1997). A membrane sequencing batch reactor performed better than a sequencing batch reactor in treating beam house wastewater that was collected after the oxidation of sulfide compounds. The former reactor achieved a removal efficiency of 100% for ammonium ion and 90% for COD, and the latter reactor achieved low ammonium removal and 90% for COD. The sequential batch reactor exhibited a washout in 90 days, whereas the membrane bioreactor was very stable for more than 120 days of operation (Martinez et al., 2003; Goltara et al., 2003). A settled vegetable-tanning process effluent was treated successfully in a mixed continuous-flow laboratory-scale plant. The BOD and COD removal efficiencies were 85 to 96% and 86 to 97%, respectively, under steady-state conditions (Murugesan and Elangovan, 1989). Activated-sludge treatment of chrome-tanning waste mixed with sewage was able to remove 87 to 96% of BOD under steady state conditions (Murugesan et al., 1996). About 84 to 92 % of the influent BOD was removed from a tannery effluent when it was treated in an activated sludge well mixed reactor (Ram et al., 1999; Orhon et al., 1999). A comparative study of tannery waste treatment was done using an upflow anaerobic sludge blanket reactor (UASB) and activated sludge (AS) reactor; interestingly, the latter was found to have more advantages than the former with respect to the capital and operating costs as well as the quality of performance (Tare et al., 2003). Total annualized costs, including
138 Biotreatment of Industrial Effluents capital, operating, and maintenance costs, for the UASB and AS plants were Indian Rs. 4.24 million/million liters per day (MLD) and Indian Rs. 3.36 million/MLD, respectively. The treated UASB effluent had higher BOD and COD and considerable amounts of chromium and sulfide when compared with the AS reactor effluent. Anaerobic T r e a t m e n t Although the anaerobic treatment of tannery beam house wastewater looks attractive, the presence of sulfide, namely hydrogen sulfide, inhibits methanogenic bacteria. Undissociated H2S is toxic, since it diffuses "freely" through the cell membrane, denatures proteins and enzymes (sulfide crosslinking), and affects internal cell pH. In a continuous-flow fixed-film reactor, a concentration of 100 mg/L of undissociated sulfide inhibited the efficiency and degree of degradation, which was eliminated by incorporating a sulfidestripping system that reduced the concentration of undissociated sulfide to 30 mg/L. This modification improved the efficiency of the degradation by 15%. Acidogenic bacteria were not inhibited by hydrogen sulfide (Schenka et al., 1999). A combined anaerobic and aerobic treatment of tannery wastewater effluent containing 900 mg/L organic carbon content (DOC) gave a removal efficiency of 85 % (Reemtsma, and Jekel, 1997). Under anaerobic conditions, sulfate is converted by sulfate-reducing bacteria (SRB) into sulfide, which is not only a toxic compound but also a strong inhibitor of methanogenesis. Also, SRB compete with methane producing bacteria (MPB) for substrates such as hydrogen and acetate and with syntrophic acetogenic bacteria (SAB) for intermediate substrates such as short-chain volatile fatty acids (VFA) and alcohols. This results in a reduction of organics available for conversion to methane. So it is advisable to remove the sulfide either before the anaerobic treatment or as part of the biotreatment cycle as shown in Fig. 12-2. A combination of the sulfur recovery unit integrated with a USAB reactor for treating tannery effluent led to improved biogas production and also recovery of elemental sulfur (Suthanthararajan et al., 2004). The sulfur removal unit consisted of a stripper column, absorber column, regeneration unit, and sulfur separator. A stripper efficiency of about 65 to 95 % in terms of sulfide removal was achieved. The efficiency of degradation in a continuous flow fixed film reactor improved by 15 % when the concentration of undissociated sulfide was reduced from 100 to 30 mg/L with the help of a continuous sulfide removal system (Wiemanna et al., 1998).
Chromium Chromium is an important heavy metal used in the leather, electroplating and metallurgical industries. More than 170,000 tonnes of chromium wastes are discharged into the environment. In India 700,000 tons of wet salted hides
Sulfide production prevented and methanogenisis
FIGURE 12-2. Sulfide handling.
and skins are processed annually in about 3,000 tanneries that discharge 3 x 107L of wastewater with suspended solids in the range of 3,000 to 5,000 mg/L and chromium as Cr in the range of 100 to 200 mg/L (1999 data). Some 200,000 tons of partly dried (40% dry matter) chromium-containing sludge is generated and is dewatered mostly in pen sludge drying beds using sand and gravel, a chamber filter press, or a belt press. The moisture content of the dewatered sludge from sludge drying beds ranges from 50 to 70% and the concentration of chromium as Cr ranges from 1 to 3 % on a dry solid weight basis. The disposal of such large quantities of hazardous solid chromium waste poses serious environmental and health problems (Rajamani et al.,
2000). The earliest technique practiced for the disposal of chromium sludge consisted of solidification of the waste with cement and organic clay. The acidic ion exchange resins Amberlite IRC 76 and Amberlite IRC 718 retained 95 % of the chromium at pH 5 (Kocaoba and Akcin, 2002). Extraction using supported liquid membranes (Djane et al., 1999), chemical methods such as reduction by sodium metabisulfite, ferrous ions, zero valent iron, and a mixture of dimethyl dithiocarbamate, ferrous sulfate, and aluminum chloride have also been tested. The last method is also practiced commercially (Chang, 2003). Several heterotrophs and coliforms were tolerant to a chromate level of greater than 50 g/mL, and many coliforms were resistant to higher levels of chromate too, whereas only a few heterotrophs were resistant to Cr 6+ at a level of greater than 150 g/mL (Vermaa et al., 2001). A few important microbes involved in the reduction of chromium are Pseudomonas aeruginosa, Enterobacter cloacae, and P. fluorescens. Desulfovibrio desulfuricans
140 Biotreatment of Industrial Effluents immobilized on a polyacrylamide gel reduced 80% of 0.5 M Cr (VI)with lactate o r H2 as the electron donor and Cr (VI) as the electron acceptor (Kamaludeen et al., 2003a). NCIM 5080 and NCIM 5109 actinomycetes strains have been found to reduce chromium levels by 99% within 24 h and at the same time reducing 70 to 80% of the COD in 74 to 96 h (Kumar, 2003). Two strains, Bacillus circulans and B. megaterium, were able to bioaccumulate 34.5 and 32.0 mg Cr per gram of dry weight, respectively, and decrease Cr(VI)concentration from 50 mg/L to less than 0.1 mg/L in 24 h. Living and dead cells of B. coagulans biosorbed 23.8 and 39.9 mg Cr per gram of dry weight, respectively, and living and dead cells of B. megaterium biosorbed 15.7 and 30.7 mg Cr per gram of dry weight, respectively (Srinatha et al., 2002). Biosorption by the dead cells was higher than that of the living cells due to pH conditioning of the dead cells. Microbes that were able to biosorb chromium include Oscillatoria sp., Arthrobacter sp., Agrobacterium sp., Pseudomonas aeruginosa 5128, and sulfate-reducing bacteria. Pretreatment enhances the biosorption capacity as seen in the case of dead Rhizopus nigricans. Chromium uptake varied, depending on the type of pretreatment at pH of 2.0. Biosorbent efficiency decreased when the microorganisms were treated with mild alkali, while treatment with acids, alcohols, and acetone improved the chromium uptake capacity (Bai and Abraham, 2002). The waste Mucor meihi biomass was found to be an effective biosorbent for the removal of chromium from industrial tanning effluents, reaching sorption levels of 1.15 mmol/g (Tobin and Roux, 1998). Dried and classified Pinus sylvestris bark was able to remove 90% of trivalent chromium. Pretreatment of the bark helped to increase its chromium sorption capacity (Alves et al., 1993). A column packed with calcium alginate (CA) beads with humic acid could adsorb 54 % of the chromium from a tannery effluent and also reduce its ecotoxicity in 72 h of operation (Pandeya, 2003). Dunaliella, a unicellular biflagellate halophilic green algae, biosorbed 45 to 58 mg/g Cr(VI). The green algae Carlina vulgaris, Scenedesmus obliquus, Synechocystis sp., Cladonia crispata, and Spirogyra sp., had maximum uptakes of 33.8, 30.2, 39.0, 39.5, and 14.7 mg/g, respectively. Fungal species of Mucor meihi, Rhizopus nigricans, R. arrhizus, and, and Aspergillus niger biosorbed 59.8, 119.7, 58.1, and 15.6 mg Cr/g, respectively (Donmez and Aksu, 2002). A strain of Streptomyces griseus was found to grow in glucose/sodium acetate medium and reduce Cr 6+ to Cr 3+ (Laxman and More, 2002).
Conclusions Figure 12-3 lists the various physical, chemical, and biochemical methods that have been tried for treating tannery effluents. Several issues such as the cost of physical and chemical methods, the toxicity of chromium on
FIGURE 12-3. Physical, chemical, and biochemical t r e a t m e n t techniques for tannery effluent.
biochemical methods, and the inhibitory nature of sulfides in the anaerobic degradation process have not been fully resolved. The reduction and recycle of the various streams at the source appears to be a good approach to dramatically decrease the present quality and quantity of effluent generated by this industry. Disposal of the sludge after biochemical treatment, however, has not been satisfactorily solved. Biochemical treatment of chromium is also discussed in Chapter 13, Treatment of Waste from Metal Processing and Electrochemical Industries. References Alves, M. M., C. G. Gonz~ilez Beta, R. Guedes de Carvalho, J. M. Castanheiral, M. C. Sol Pereira, and L. A. T. Vasconcelos. 1993. Chromium removal in tannery wastewaters "polishing" by Pinus sylvestris bark. Water Res. 27(8):1333-1338. Bai, S. R., and T. E. Abraham. 2002. Studies on enhancement of Cr(VI) biosorption by chemically modified biomass of Rhizopus nigricans. Water Res. 36:1224-1236. Cassano, A., J. Adzet, R. Molinari, M. G. Buonomenna, J. Roig, and E. Drioli. 2003. Membrane treatment by nanofiltration of exhausted vegetable tannin liquors from the leather industry. Water Res. 37:2426-2434. Chang, L. Y. 2003. Alternate chromium reduction and heavy metal precipitation methods for industrial wastewater. Environ. Prog. 22(3):174-182. Djane, N. K., K. Ndung'u, C. Johnsson, H. Sartz, T. Tornstrom, and L. Mathiasson. 1999. Chromium speciation in natural waters using serially connected supported liquid membranes. Talanta 48:1121-1132.
B i o t r e a t m e n t of I n d u s t r i a l E f f l u e n t s
Donmez, G., and Z. Aksu. 2002. Removal of chromium (VI) from saline wastewaters by Dunaliella species. Process Biochem. 38:751-762. Goltara, A., J. Martinez, and R. Mendez, 2003. Carbon and nitrogen removal from tannery wastewater with a membrane bioreactor. Water Sci. Technol. 48(1):207-214. Iaconi, C. D., A. Lopeza, R. Ramadoria, A. C. Di Pintob, and R. Passino. 2002. Combined chemical and biological degradation of tannery wastewater by a periodic submerged filter (SBBR). Water Res. 36:2205-2214. Jochimsena, J. C., H. Schenkb, M. R. Jekela, and W. Hegemann. 1997. Combined oxidative and biological treatment for separated streams of tannery wastewater. Water Sci. Technol. 36(2-3):209-216. Kabdasli, I., T. Olmez, and O. Tiinay. 2003. Nitrogen removal from tannery wastewater by protein recovery. Water Sci. Technol. 48(1 ):215-223. Kamaludeen, S. P. B., K R A. Kumar, S. Avudainayagam, and K. Ramasamy. 2003a. Bioremediation of chromium contaminated effluents. Indian J. Exper. Biol. 41(9):972-985. Kamaludeen, S. P. B., M. Megharaj, R. Naidu, I. Singleton, A. L. Juhasz, B. G. Hawke, and N. Sethunathan. 2003b. Microbial activity and phospholipid fatty acid pattern in long-term tannery waste-contaminated soil. Ecotoxicol. Environ. Safe. 56:302-310. Kocaoba, S., and G. Akcin. 2002. Removal and recovery of chromium and chromium speciation with MINTEQA2. Talanta 57:23-30. Kumar, A. 2003. Chromium-chomping bacteria to clean tannery waste. Terra Green 36:May 15. Laxman, R. S., and S. More. 2002. Reduction of hexavalent chromium by Streptomyces griseus. Minerals Eng. 15:831-837. Macchi, G., M. Pagano, M. Pettine, M. Santori, and G. Tiravanti. 1991. A bench study on chromium recovery from tannery sludge. Water Res. 25(8):1019-1026. Martinez, J. M., A. Goltara, and R. Mendez. 2003. Tannery wastewater treatment: comparison between SBR and MSBR. Water Supply 3(5):275-282. Murugesan, V., B. Arabindoo, and R. Elangovan. 1996. Treatability studies and evaluation of biokinetic parameters for chrome tanning wastewater admixtured with sewage. J. Ind. Pollution Control 12( 1):41-53. Murugesan, V., and R. Elangovan. 1989. Biokinetic parameters for activated sludge treating vegetable tannery waste. Indian J. Environ. Protection 14:511-515. Orhon, D., O. Karahan, and S. Sozen. 1999. The effect of residual microbial products on the experimental assessment of the particulate inert cod in wastewaters. Water Res. 33( 14):3191-3203. Pandeya, K., S. D. Pandeya, V. Misra, and A. K. Srimal. 2003. Removal of chromium and reduction of toxicity to Microtox system from tannery effluent by the use of calcium alginate beads containing humic acid. Chemosphere 51(4):329-333. Rajamani, S., E. Ravindranath, R. S. Rajan, K. Chitra, B. U. Maheswari, and T. Ramasami. 2000. Generation of hazardous sludge from tannery effluent treatment plants and disposal problems in India, in 2000 Pacific Basin Conference, Manila, Philippines, April 12-14. University of the Philippines: Pacific Basin Consortium for Hazardous Waste Research and Management. Ram, B., P. K. Bajpai, and H. K. Parwana. 1999. Kinetics of chrome-tannery effluent treatment by the activated-sludge system. Process Biochem. 35:255-265. Reemtsma, T., and M. Jekel. 1997. Dissolved organics in tannery wastewaters and their alteration by a combined anaerobic and aerobic treatment. Water Res. 31(5):1035-1046. Schenka, H., M. Wiemannb, and W. Hegemannc. 1999. Improvement of anaerobic treatment of tannery beamhouse wastewater by an integrated sulphide elimination process. Water Sci. Technol. 40(1):245-252. Scholz, W., and M. Lucas. 2003. Techno-economic evaluation of membrane filtration for the recovery and re-use of tanning chemicals. Water Res. 37:1859-1867. Srinatha, T., T. Vermaa, P. W. Ramteke, and S. K. Garg. 2002. Chromium (VI) biosorption and bioaccumulation by chromate resistant bacteria. Chemosphere 48(4):427-435. Suthanthararajan, R., K. Chitra, E. Ravindranath, B. Umamaheswari, S. Rajamani, and T. Ramesh. 2004. Anaerobic treatment of tannery wastewater with sulfide removal and recovery of sulfur from wastewater and biogas. J. Amer. Leather Chemists 99(2):67-72.
Tare, V., S. Gupta, and P. Bose. 2003. Case studies on biological treatment of tannery effluents in India. J. Air & Waste Manage. Assoc. 53:976-982. Tobin, J. M., and J. C. Roux. 1998. Mucor biosorbent for chromium removal from tanning effluent. Water Res. 32(5):1407-1416. Vermaa, T., T. Srinatha, R. U. Gadpaylea, P. W. Ramteke, R. K. Hansa, and S. K. Garg. 2001. Chromate tolerant bacteria isolated from tannery effluent. Bioresource Technol. 78( 1):31-35. Wiemanna, M., H. Schenka and W. Hegemanna. 1998. Anaerobic treatment of tannery wastewater with simultaneous sulphide elimination. Water Res. 32(3):774-780. Zengin, G., T. Olmez, S. Dogcruel, I. Kabda~l~, and O. Tiinay. 2002. Assessment of sourcebased nitrogen removal alternatives in leather tanning industry wastewater. Wat. Sci. Tech. 45(12):205-215.
Bibliography Espantaleon, A. G., J. A. Nieto, M. Fernandez, and A. Marsal. 2003. Use of activated clays in the removal of dyes and surfactants from tannery waste waters. Appl. Clay Sci. 24:105-110. Gokcay, C. F., and U. Yetis. 1991. Effect of chromium (VI)on activated sludge. Water Res. 25(1):65-73. Lawrence, A. W., and P. L. McCarty. 1970. Unified basis for biological treatment design and operation. J. Sanit. Eng. Div. Am. Soc. Civil Engr. 96(SA3):757-778. Orhon, D., E. A. Genceli, and S. S6zen. 2000. Experimental evaluation of the nitrification kinetics for tannery wastewaters. Water SA 26(1):43-49. Rossini, M., J. Garcia Garrido and M. Galluzzo. 1999. Optimization of the coagulationflocculation treatment: influence of rapid mix parameters. Water Res. 33(8):1817-1826. Schrank, S. G., H.J. Jos6, and R. F. P. M. Moreira. 2002. Simultaneous photocatalytic Cr(VI) reduction and dye oxidation in a TiO2 slurry reactor. J. Photochem. Photobiol. A: Chemistry 147:71-76. Song, Z., C. J. Williams, and R. G. J. Edyvean. 2000. Sedimentation of tannery wastewater, Water Res. 34(7):2171-2176.